1. Field of The Invention
The present invention relates to a negative pressure control apparatus for a brake booster, and particularly to a negative pressure control apparatus for controlling negative pressure in a brake booster in a system which has a function of increasing a brake fluid pressure by increasing the negative pressure of the brake booster.
2. Description of the Related Art
Conventionally, as disclosed in Japanese Laid-Open Patent Application No. 5-208663, a braking apparatus having a brake booster is known. The brake booster is a mechanism which assists a brake operation by using an intake manifold negative pressure as a power source so that a larger braking force can be generated. The above braking apparatus has a function of increasing a master cylinder pressure by increasing negative pressure of the brake booster when a brake operation is not being performed. Hereinafter, this function is referred to as a booster charge function.
The above braking apparatus also has a function of performing an automatic brake control for generating a braking force when an obstacle is detected ahead of the vehicle in a situation where no brake operation is being performed. This automatic brake control is achieved by increasing the master cylinder pressure using the above-mentioned booster charge function. Additionally, the above braking apparatus increases the negative pressure of the brake booster toward a predetermined target value when execution of the automatic brake control is predicted so as to positively perform the brake control.
However, when a temperature of brake fluid is low, a delay occurs in transmitting a fluid pressure to wheel cylinders due to an increase in viscous resistance of the brake fluid. Thus, in the above-mentioned conventional braking apparatus, a response of the automatic brake control controlling the brake fluid pressure may be delayed when the temperature is low.